1. Field of the Invention
The present invention relates to an electromagnetic device and, more particularly, to an electromagnetic device suited for use in a camera.
2. Description of the Prior Art
In general, a conventional electromagnetic device is constructed as shown in FIG. 1, that is, from a ground plate 1, a yoke 2 fixedly mounted thereon, and coils 4 wound on the yoke 2 through bobbins 3, with these coils 4 being connected through the ends 2a to an electromagnet control circuit (not shown). In a portion of the magnetic path of the above-described yoke 2, there is provided a permanent magnet 2b for attraction, arranged, for example, as shown in FIG. 2. The device further includes an armature 5 arranged to be normally attracted to the yoke 2 by magnetic force and upon energization of the coils 4 to be moved away from the yoke 2 by the biasing force of a spring 7 on an armature lever 6.
In such a electromagnetic device, it is desired that the adhesion stability between the yoke 2 and armature 5 be excellent. Particularly in application to photographic cameras and the like, an electromagnetic device which lacks sufficient adhesion stability leads to faulty operation, finally resulting in improper exposures. For this reason, there has been an improved demand for such adhesion stability.
To achieve this improvement, the confronting end surfaces of the yoke 2 and armature 5 must be made precisely parallel to each other, in other words, the armature shaft 8 must be made exactly normal to the armature 5. To establish this normal arrangement of the armature shaft 8 to the armature 5, the armature shaft 8 is brought into normal relationship to the armature lever 6, and then the armature 5 has to be placed perpendicular to this shaft. To do so, however, there is some difficulty with respect to machining accuracy. Accordingly, it follows that in practice, the armature shaft 8 tends to be set with some inclination to the armature plane 5. Therefore, it is very difficult to obtain the desired precisely parallel arrangement between the confronting end surfaces of the yoke 2 and armature 5, that is, to assure good stability of adhesion.
If the end surface of the yoke 2 is not parallel to that of the armature 5, when the armature 5 is brought into adhesion with the yoke 2 by the magnetic force, the armature 5 will adhere to the yoke 2 in an inclined position where the adhesion is not sufficiently stabilized. Alternatively, when the armature 5 is moved away from the yoke 2 under the action of the spring 7, a force inclined to the direction in which the armature 5 moves is also applied to the armature so that even a smaller force is able to break the adhesion of the armature 5 to the yoke 2. Therefore, sufficient adhesion is not provided. Thus, since the actuation for movement of the armature can be effected by a weak force, any shock, however small, when applied to the electromagnetic device from outside, will cause accidental separation of the armature 5 from the yoke 2. Accordingly, the use of such electromagnetic device in a camera gives rise to faulty operation which, in turn, will very likely cause improper exposures.
According to the prior art, the above-described drawback of the conventional electromagnetic device, i.e., the problem of improving the adhesion stability, has been overcome by employing such an arrangement of the armature 5 and the armature lever 6 as shown in FIG. 3. That is, the armature shaft 8 is provided with a projected portion 8a in the form of a ring of semi circular cross-section around the central peripheral portion thereof and a flanged portion 8d, and is fixedly mounted on the armature lever 6 at one end 8b thereof. The armature 5 is rotatably and tiltably fitted at a penetration hole 5a thereof on the projected portion 8a, and is restrained from outward axial movement by an E shaped ring 10 engaging in a circumferentially grooved position 8c near the other end thereof. By such construction, the possibility that the armature 5 and the yoke 2 will not be parallel can be reduced so that the armature 5 adheres to the yoke 2 without causing excessive inclination. Thus, the degree of stability of adhesion is increased. Also, when the armature 5 is moved away from the yoke 2, a twist-like motion can be eliminated so that the armature is protected even to external shocks, and accidental separation of the armature 5 is not easily effected.
However, even in such conventional type of electromagnetic devices, there was failure in establishing parallelism between two separate areas of the active surface of the yoke, there was insufficient stability of adhesion, and the device was not sufficiently stabilized to external shocks. Further, since the ring-like projection 8a of the armature shaft 8 contacts with the inner surface of the break-through hole 5a in a line, frequent repetition of actuation, that is, attraction and release, results in a large amount of abrasion at the contact areas. Accordingly, the adhesion stability tends to deteriorate and the resistance to external shock also tends to be weakened.
Another disadvantage in this prior contruction is that, because of the provision of a leaf spring 11 between the armature 5 and a bent-off portion of the armature lever 6 to increase the stability of adhesion, when the armature is removed from contact with the yoke 2, vibrations about the armature shaft 8 by the leaf spring 11 are imparted to the armature 5. If this operation is repeated at a high frequency as in motor driven photography, the next charging operation is initiated before the vibrations have been damped to a sufficiently low level, thereby the confronting end surfaces of the armature 5 and yoke 2 are damaged to effect a substantial reduction in adhesion force. This constitutes another problem concerning the durability of the device.